The present invention relates to a hydraulic cylinder, and particularly to an improvement in a single-acting type hydraulic cylinder suitable for use with a forklift.
In the past, a single-acting type hydraulic cylinder for a forklift is operated to be extended, for example, as shown in FIG. 2, when liquid pressure P is supplied from the outside to a liquid chamber R which is a chamber on the counter rod side partitioned by a piston 2 slidably inserted into a cylinder body 1.
Further, the hydraulic cylinder is operated to be contracted when a supply of liquid pressure P to the liquid chamber R is released whereas a rod body 3 having a proximal end connected to the piston 2 gets into the cylinder body 1 due to own weight or load exerting on the extreme end to contract the liquid chamber R through the piston 2.
At this time, the rod side chamber partitioned by the piston 2 within the cylinder body 1 which is an air chamber A is in many cases communicated with the atmosphere outside the cylinder body 1 through a communication hole 1a bored in the cylinder body 1.
On the other hand, the rod body 3 which moves in and out of the cylinder body 1 extends through an axial part of a head member 4 forming an open end of the cylinder body 1. At this time, a bushing 5 for causing the outer circumference of the rod body 3 to slidingly contact therewith is disposed in the inner circumference internally of the cylinder body 1 in the head member 4, and a dust seal 6 for causing the outer circumference of the rod body 3 to slidingly contact therewith is disposed in the inner circumference of an open end in the head member 4.
Therefore, in the hydraulic cylinder, for example, when it is used for a forklift, a fork loaded with baggage can be moved up by the extension action thereof, and the fork loaded with baggage can be moved down by the contraction action thereof. At this time, when the rod body 3 is moved in and out of the cylinder body 1, the bushing 5 compensates for slidability of the rod body 3 with respect to the head member 4 whereas the dust seal 6 wipes out so-called dust adhered to the outer circumference of the rod body 3 so that dust is not drawn into the cylinder body 1.
However, the conventional hydraulic cylinder shown in FIG. 2 involves a fear that when the rod body 3 is moved in and out of the cylinder body 1, so-called heat generation occurs, and rust, discoloration and scratch due to the beat generation appear on the outer circumference of the rod body 3.
That is, in the above-described hydraulic cylinder, since the rod side chamber partitioned by the piston 3 within the cylinder body 1 comprises the air chamber A, a so-called xe2x80x9cdryxe2x80x9d condition is formed between the inner circumference of the bushing 5 and the outer circumference of the rod body 3, tending to poor lubrication as compared with the case of a so-called xe2x80x9cwetxe2x80x9d condition where an oily sliding coating is formed.
Therefore, when sliding movement is frequently repeated between the rod body 3 and the bushing 5, particularly when sliding movement such that the rod body 3 is pressed against the bushing 5 is repeated, a heat generation phenomenon tends to appear between the inner circumference of the bushing 5 and the outer circumference of the rod body 3.
Then, when the heat generation phenomenon appears, a degeneration is brought forth in the outer circumference of the rod body 3 due to the heat generation, and rust or discoloration appears on the outer circumference of the rod body 3 due to the degeneration.
At this time, also in the bushing 5, abrasion is accelerated to deteriorate a function of the bushing, and scratching occurs on the outer circumference of the rod body 3, resulting in damage of even the dust seal 6.
As a consequence, occurrence of rust or formation of scratches in the outer circumference of the rod body 3 further lowers the so-called slidability of the rod body 3 with respect to the cylinder body 1, posing an inconvenience making it impossible to expect the desired permanent operation of the hydraulic cylinder.
The present invention has been created in view of the above described circumstances, and has its object to provide a single-acting type hydraulic cylinder which is operated to be expanded and contracted by supply and discharge of liquid pressure from the so-called bottom side, the hydraulic cylinder being optimum for permanently securing the desired operation without bringing forth abrasion of a bushing and damage of a dust seal caused by rust or scratching in the outer circumference of a rod body.
For achieving the aforementioned object, the present invention provides a hydraulic cylinder fundamentally constituted such that the cylinder is operated to be extended when liquid pressure from outside is supplied to a liquid chamber which is a counter rod side chamber partitioned by a piston slidably inserted into a cylinder body. The cylinder is operated to be contracted when a supply of liquid pressure to the liquid chamber is released whereas a rod body having a proximal end connected to a piston is moved into the cylinder body due to its own weight or load exerting on the extreme end. A head member forming an open end of a cylinder body for moving in and out of the rod body has a bushing for causing the outer circumference of the rod body to sliding contact therewith and a dust seal. The hydraulic cylinder, in the head member, comprising in series, a lubrication means in sliding contact with the outer circumference of the rod body on the bushing side between the bushing and the dust seal, and an oil seal in sliding contact with the outer circumference of the rod body on the dust seal side.
In the above-described constitution, more concretely, the lubrication means is for example, a lubricating material such as grease stored in an inner circumferential groove at a predetermined position formed in the inner circumference of the head member, or a suitable impregnate material received in the inner circumferential groove, the impregnate material being impregnated with a suitable lubricating material.
Further, the rod side chamber partitioned by a piston within the cylinder body comprising an air chamber is communicated with the atmosphere outside the cylinder body through a communication hole bored in the cylinder body, or is set to an air spring chamber having a seal construction not in communication with the atmosphere outside the cylinder body while being an air chamber.